In a typical proton therapy system used for tumor radiation treatment for example, a proton beam is generated and output from an accelerator, e.g., a cyclotron or a synchrotron, with a certain initial energy. The initial energy determines a maximum penetration depth of the proton beam and typically is 250 MeV. As the proton beam travels through a beam transportation system or a beamline, the beam energy is precisely tuned through energy selection mechanisms, e.g., an energy degrader or energy slit. The beam transport system includes a plurality of magnets for beam redirection (bending), focusing and steering. A rotational gantry with a radiation nozzle is located at the end of the beam transport system. Eventually, the beam is delivered to a treatment station and irradiated onto a patient at an energy level prescribed for the specific treatment session based on the tumor volume, geometry, location and etc.
Due to the extremely high cost for purchasing and maintaining such a radiation system, a medical facility usually uses one accelerator for a plurality of treatment stations so the high expenditure for the accelerator facilities is distributed. Although using a multi-station single-cyclotron system is effective to distribute the cost for large medical facilities, the overall cost for such a multi-gantry system can be prohibitively high for smaller healthcare facilities that may only need one treatment station. Also, some multi-station systems do not support simultaneous treatment in multiple stations. This contribute to further disadvantage that a delay at one treatment station can cause delay at the other station.
With the demand for proton beam radiation therapy rising worldwide, smaller and less expensive proton therapy systems are highly desired to increase patient access to therapy. In a proton radiation system, a gantry system alone typically weighs over 200 tons which is mainly contributed by the massive magnets installed in the gantry. To support and precisely control the motion of a tremendous amount of weight, existing rotatable gantries are supported by a front and a rear ring structures, between which the components in the gantry beamline are suspended. That is, the gantry is supported in a simply-supported manner.
FIG. 1 illustrates the supporting mechanism of a simply-supported gantry 100 in a proton beam therapy system according to the prior art. The gantry 100 includes beam optics (or a beamline) coupled to a beam nozzle 110. A series of magnets (not shown) in the beamline operate to guide the transportation of a proton beam which eventually exits from the beam nozzle 110 and is irradiated onto a patient. The gantry 100 rotates around the iso-center 101 which defines the positioning of a treatment table and a patient during operation.
Supported in a simply-supported manner, the weight of the gantry 100 is supported at both end portions of the gantry (as illustrated by the dotted circles 121 and 122). More specifically, a front ring 120 and a rear ring 150 are respectively coupled to the front and rear ends of the gantry 100. The front and rear rings 120 and 150 are respectively coupled to the front and rear rollers 161 and 162 that can rotate with the gantry 100. The rings and rollers are affixed to the ground through fixtures 171 and 172 which are made of steel and concrete.
The two rings 130 and 150, plus the additional structural members to stiffen the assembly, make the gantry system appear to be a tremendously large conical (generally tapering from right to left as illustrated) or cylindrical drum-shaped structure in a treatment station. The drum-shaped structure defines the overall size of the gantry, such as the end-to-end gantry length 102. Such a drum-shaped gantry undesirably limits patient positioning and makes it difficult to make incremental improvements to the beam optics geometry because the critical dimensions are determined by the mechanical structure, not by the magnet positioning.
More importantly, using two rollers to support introduces random deflection errors during rotation due to the inherent difficulty in aligning the two rollers perfectly. As a result, the gantry tends to wobble causing the beam spot location to shift in an unpredictable manner. In practice, remarkable time and resources are usually spent on realigning and repositioning the rollers in the attempt to fix the random errors and maintain beam precision.